1. Field of the Invention
The present invention relates to a method and an apparatus for detecting an information rate of information data in communications in which the information rate is variable.
2. Description of the Related Art
As Allen Salmasi et al. describe in their literature, "On The System Design Aspects Of Code Division Multiple Access (CDMA) Applied To Digital Cellular And Personal Communications Networks (1991 41st IEEE Vehicular Technology Conference)", a variable rate voice CODEC has been used in the CDMA system for the purpose of reducing power consumption of a mobile unit and reducing interference to other mobile units and the base station. The voice CODEC codes and decodes signals in a frame unit, so that an information rate (amount of information) of the coded data changes frame to frame. At the signal receiving side, it is necessary to decode the signal after detecting the information rate. In general, data of voice CODEC is transmitted after an error-correcting coding and other processing.
According to the prior-art mobile unit, a transmitter section provides transmission data having a constant transmission rate by arranging coded data repeatedly by the number corresponding to the current rate. The coded data are formed by convolution coding on the information data coded through the variable rate voice CODEC. A receiver section for receiving the transmitted data can not decode the transmission data correctly if the receiver does not know the exact rate at which the transmitter has coded the information data. Therefore, the receiver section obtains decoded results by Viterbi decoding the received data at every possible information rate, performs convolution coding on each of the decoded results at each corresponding rate again, collates the convolution coded data with the received data, decides the rate used in the convolution coding of the information data by the transmitter section based on the convolution re-coded data which is the most coincident with the received data of the receiver section, and then outputs the decoded result which corresponds to this rate as Viterbi decoded data. A literature, "CDMA Mobile Station Modem ASIC, (IEEE Journal Of Solid-State Circuits, Vol. 28, No. 3, March, 1993)" discloses a method for detecting an information rate according to the method described above.
As described above, the conventional method can detect an information rate in variable rate communications, by Viterbi decoding received data and performing convolution coding on the decoded result again at all potential information rates, and estimating the information rate.
The above-described conventional method for detecting an information rate, however, has problems in that it is necessary to estimate all possible information rates and apply a Viterbi decoding, which is known to require a large change of arithmetic operations, to the respective information rates, so that it is necessary to increase the hardware capacity and increase the circuit operation speed in order to complete the processing within a limited time available, and in that the above requirements lead to an increase in the power consumption at mobile terminal units which are driven by batteries.